1. Field of Use
This invention relates generally to material chopping devices. In particular, it relates to a knife assembly for use in a chopping apparatus comprising a rotatable disc.
2. Description of the Prior Art
Knife assemblies for material chopping devices are of various types and include assemblies for engaging conventional one-edged knives and assemblies for the usually smaller, two-edged reversible knives. Such reversible knives have several advantages over their one-edged counterparts, particularly in their ease of handling, but suffer from some problems, whose remedies make the blades somewhat expensive.
Such knives and their assemblies are typically shown in U.S. Pat. No. 4,047,670, issued to Svensson, on Sept. 13, 1977, and entitled "Knife Device for a Chopper". The knife shown in FIGS. 2 and 3 therein is wedged between a first and a second knife holding means. The knife includes a pair of flat contact surfaces forming an angle, or being coangular, with each other and resting against corresponding contact surfaces of the first knife holding means, and also includes another pair of flat contact surfaces forming an angle with each other and resting against corresponding contact surfaces of the second knife holding means. Good contact and fit of the knives to the knife holding means is essential to ensure adequate heat transfer from the knives when the chopping apparatus is being operated and also to ensure that small pieces of the chopped material do not enter spaces created by a poor fit between the knives and the holding means. Good contact between the knives and the holding means in a knife having the flat and angled contact surfaces on each of its two sides as shown in Svensson requires that those knife contact surfaces be machined to very small and critical tolerances. If the angle between the knife's flat contact surfaces is off of its specification by only a small amount, at least one of the knife's flat contact surfaces will not abut a substantial part of the corresponding contact surface of the knife holding means. Thus, good heat transfer away from such knives and to the holding means can only be assured by maintaining strict manufacturing standards, an expensive requirement.
A non-coangular or straight knife contact surface would transfer the lowest stress concentrations to the knife holding means. Thus, by making the contact surfaces coangular, a higher stress is placed on the knife holding means and those means must then be constructed of a harder metal, and metal hardness is an important factor in such machine design.
The knives impact the material to be chopped, such as wood, at linear knife speeds of up to eighty miles per hour and are driven by considerable horsepower. As the knives impact the wood, tremendous forces are transmitted to those knives and transferred to the knife holding means. The smaller the area over which these forces are transmitted, the greater the force per unit area on the knife holding means and the harder and more expensive the metal of which they are constructed must be. The knives of some prior art devices have flat surfaces engaging both the first and second knife holding means, and flat surfaces offer very little area between adjacent workpieces. The importance of precisely maintained angles between the knife's flat contact surfaces forming an angle with each other is evident here as well, because imprecisely machined angles lower the area of the knife contacting the knife holding means and thus require the knife holding means to be constructed of metals stronger than would be necessary with properly machined angles.
The knife holding means, particularly the knife holding means adjacent the opening by which the chopped material passes through and away from the disc, is subjected to large amounts of thrust from the knife. That knife holding means thus acts as a thrust-bearing element and, as other thrust-bearing elements, wears after extended use and then requires replacement.